The market for PET refillable carbonated soft drink (CSD) bottles has enjoyed significant growth worldwide since its introduction in 1987 by Continental PET Technologies. These bottles have been commercialized throughout much of Europe, Central and South America, and are now moving into the Far East market.
Refillable bottles reduce the existing landfill and recycle problems associated with disposable plastic beverage bottles. In addition, a refillable bottle provides a safer, lighter-weight plastic container in those markets, currently dominated by glass, where legislation prohibits use of non-returnable packages. The goal is to produce a refillable bottle having the necessary physical characteristics to withstand numerous refill cycles, and which is still economical to produce.
Generally, a refillable plastic bottle must maintain its functional and aesthetic features over a minimum of 10 and preferably over 20 cycles or loops to be considered economically feasible. A loop is comprised of (1) an empty hot caustic wash followed by (2) contaminant inspection and product filling/capping, (3) warehouse storage, (4) distribution to wholesale and retail locations and (5) purchase, use and empty storage by the consumer followed by eventual return to the bottler. This cycle is illustrated in FIG. 1. In an alternative cycle, the contaminant inspection occurs prior to the caustic wash.
Refillable containers must meet several key performance criteria to achieve commercial viability, including:                1. high clarity (transparency) to permit on-line visual inspection;        2. dimensional stability over the life of the container; and        3. resistance to caustic wash induced stress cracking and leakage.        
A commercially successful PET refillable CSD container is presently being distributed by The Coca-Cola Company in Europe (hereinafter “the prior art container”). This container is formed of a single layer of a polyethylene terephthalate (PET) copolymer having 3-5% comonomer, such as 1,4-cyclohexanedimethanol (CHDM) or isophthalic acid (IPA). The preform, from which this bottle is stretch blow molded, has a sidewall thickness on the order of 5-7 mm, or about 2-2.5 times that of a preform for a disposable one-way bottle. This provides a greater average bottle sidewall thickness (i.e., 0.5-0.7 mm) required for abuse resistance and dimensional stability, based on a planar stretch ratio of about 10:1. The average crystallinity in the panel (cylindrical sidewall section beneath the label) is about 15-20%. The high copolymer content prevents visual crystallization, i.e., haze, from forming in the preform during injection molding. Preform haze is undesirable because it produces bottle haze which hinders the visual on-line inspection required of commercial refill containers. Various aspects of this prior art container are described in Continental PET Technology's U.S. Pat. Nos. 4,725,464, 4,755,404, 5,066,528 and 5,198,248.
The prior art container has a demonstrated field viability in excess of 20 refill trips at caustic wash temperatures of up to 60° C. Although successful, there exists a commercial need for an improved container that permits an increase in wash temperature of greater than 60° C., along with a reduction in product flavor carryover. The latter occurs when flavor ingredients from a first product (e.g., root beer) migrate into the bottle sidewall and subsequently permeate into a second product (e.g., club soda) on a later fill cycle, thus influencing the taste of the second product. An increase in wash temperature may also be desirable in order to increase the effectiveness and/or reduce the time of the caustic wash, and may be required with certain food products such as juice or milk.
Thus, it would be desirable to increase the caustic wash temperature above 60° C. for a returnable bottle having a lifetime of at least 10 refill trips, and preferably 20 refill trips, and to reduce the product flavor carryover. These and other objects are achieved by the present invention as set forth below.